This application is based on, and claims priority to, Japanese application 10-249658, filed Sep. 3, 1998, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical amplifier and optical amplification method for amplifying wavelength division multiplexed (WDM) light which includes light in different wavelength bands.
2. Description of the Related Art
Research and development in the area of wavelength division multiplexed (WDM) optical communication systems has resulted in a steady increase in the number of wavelengths being multiplexed together. In addition, the wavelength bands for transmission are being widened.
Furthermore, research and development is also advancing the development of WDM optical communication systems which utilize optical amplifiers as linear repeaters. With such WDM optical communication systems, a plurality of signal lights in a wavelength band of, for example, 1.53 to 1.56 xcexc(hereinafter referred to as a 1.55 xcexcm band), can be collectively amplified with an optical amplifier, thereby enabling large-capacity and long-distance light transmission with a simple construction.
In addition, optical communication systems which address band expansion of an optical amplifier have also been proposed. For example, optical amplifiers which can amplify signal lights in a long wavelength band of, for example 1.57 to 1.60 xcexcm (hereinafter referred to as a 1.58 xcexcm band) have been proposed.
For example, FIG. 1 is a diagram showing a conventional optical amplifier for amplifying WDM signal light which includes both signal light in a 1.55 xcexcm band (1.53 to 1.56 xcexcm) and signal light in a 1.58 xcexcm band (1.57 to 1.60 xcexcm). With typical optical amplifiers, in particular optical fiber amplifiers, it is difficult to obtain an equal gain over a wide band exceeding 60 nm. Therefore, the optical amplifier in FIG. 1 divides the WDM signal light into, for example, two bands of 1.55 xcexcm and 1.58 xcexcm, and obtains equal gains over the respective bands.
Referring now to FIG. 1, WDM signal light from a single optical fiber is demultiplexed by a WDM coupler 1 into WDM signal lights of a 1.55 xcexcm band and a 1.58 xcexcm band. Then, the WDM signal lights of the 1.55 xcexcm band and the 1.58 xcexcm band are directed to a 1.55 xcexcm band optical fiber amplifier section 2 and a 1.58 xcexcm band optical fiber amplifier section 3, respectively. The respective WDM signal lights amplified by optical fiber amplifier sections 2 and 3 are then multiplexed in a WDM coupler 4, and output to a single optical fiber.
However, various problems can occur with an optical communication system which transmits signal light over a wide wavelength band. For example, assume signal light of the 1.55 xcexcm band is transmitted over a long distance using an optical transmission path comprising, for example, a single mode optical fiber (SMF) which transmits the wavelength close to 1.3 xcexcm with zero dispersion. In this case, there is a problem that the transmitted waveform becomes distorted if the signal light is transmitted at a high transmission speed. This distortion is due to the wavelength dispersion characteristics of the optical transmission path.
For example, with a general 1.3 xcexcm zero dispersion SMF, there is a wavelength dispersion of approximately 18 ps/nm/km in the vicinity of 1.55 xcexcm. For example, in the case where a signal light of 1.55 xcexcm is transmitted 50 km, then a wavelength dispersion of 900 ps/nm (=18 ps/nm/kmxc3x9750 km) accumulates. This is generally referred to as primary dispersion, and indicates that a delay difference of 900 ps per wavelength amplitude of 1 nm is produced.
Whether or not this delay difference exerts an influence on the transmission characteristics is related to the time slot of the signal light. That is to say, in the case where the time slot of the signal light is sufficiently longer than the delay difference due to the wavelength dispersion, the influence on the transmission waveform is minimal. However when the time slot approaches the delay difference, the influence of the wavelength dispersion increases so that the waveform becomes distorted. In general, it is considered that if the transmission speed of the signal light per unit wavelength exceeds approximately 2.5 Gb/s, then compensation for wavelength dispersion is required. For example, in the case where the transmission speed of the signal light is 10 Gb/s, the time slot becomes 100 ps, and the wavelength dispersion of 900 ps/nm for the above mentioned case exerts a considerable influence on the transmission characteristics.
To compensate for the wavelength dispersion characteristics of the optical fiber transmission path, the light signal may be passed through a dispersion compensator having opposite wavelength dispersion characteristics to the transmission path. In the case of compensating for a wavelength dispersion of 900 ps/nm, a dispersion compensator having a wavelength dispersion of xe2x88x92900 ps/nm is used. For example, a dispersion compensating fiber (DCF) is widely used as such a dispersion compensator.
However, in the case where compensation is performed with a wavelength dispersion of 1.55 xcexcm as a reference, as the wavelength band of the signal light is increased, the compensation error increases as the deviation of the wavelength from 1.55 xcexcm increases.
For example, FIG. 2 is a diagram showing wavelength dispersion characteristics for a 1.3 xcexcm zero dispersion SMF. As shown in FIG. 2, the wavelength dispersion characteristic of a 1.3 xcexcm zero dispersion SMF has an incline with respect to wavelength. As a result, for example, a wavelength dispersion with respect to a signal light of 1.53 xcexcm becomes 18xe2x88x92xcex94S ps/nm/km, and a wavelength dispersion with respect to a signal light of 1.58 xcexcm becomes 18+xcex94L ps/nm/km. Consequently in the case where 50 km transmission is performed, then even if a dispersion compensator having a compensation amount of the abovementioned xe2x88x92900 ps/nm is used, the xcex94Sxc3x9750 ps/nm component is excessively compensated for with respect to the signal light of 1.53 xcexcm, while the xcex94Lxc3x9750 ps/nm component is insufficiently compensated for with respect to the signal light of 1.58 xcexcm. The wavelength dispersion produced due to this situation where the wavelength dispersion characteristics of the optical fiber transmission path have an incline with respect to the wavelength is generally referred to as secondary dispersion, and when the number of wavelengths of the signal light is large and the wavelength band is wide, it is necessary to perform compensation not only for primary dispersion but also for secondary dispersion.
As mentioned above, a high speed WDM optical communication system with a transmission speed per unit wavelength exceeding 2.5 Gb/s, using a 1.55 xcexcm band or a 1.58 xcexcm band as the band for wavelength division multiplexed signal light is, currently being developed. In realizing such a system, an important consideration is how to compensate for the primary and secondary wavelength dispersion to improve efficiency. Furthermore, it is considered that when wavelength dispersion compensation in the above mentioned wide wavelength band is collectively performed, a signal light of large power is transmitted to the dispersion compensator. Therefore, for example, a nonlinear optical effect such as cross-phase modulation (XPM) or four-wave mixing (FWM) is likely to occur, so that there is the likelihood of degradation of the transmission characteristics.
Accordingly, it is an object of the present invention to provide an optical amplifier and optical communication system of simple construction which can compensate for wavelength dispersion with respect to WDM signal light of a wide band. Furthermore, it is an object of the present invention to provide a wavelength dispersion compensation method which reduces the probability of the occurrence of nonlinear optical effects when transmitting WDM signal light.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing an apparatus which demultiplexes light into a plurality of wavelength bands according to optical wavelength and respectively amplifies the demultiplexed lights of the respective wavelength bands with a plurality of optical amplifiers. More specifically, the apparatus includes a plurality of optical amplifiers for amplifying the plurality of wavelength bands, a wavelength dispersion compensator for compensating for wavelength dispersion, and an optical multiplexor/demultiplexor. The optical multiplexor/demultiplexor takes out and multiplexes lights of respective wavelength bands from inside the respective optical amplifiers, and then inputs the multiplexed light into the wavelength dispersion compensator, and also demultiplexes the light output from the wavelength dispersion compensator into the respective wavelength bands and returns the demultiplexed lights to the respective optical amplifiers.
Preferably the respective optical amplifiers incorporate a pre-stage optical amplifier section and a post-stage optical amplifier section connected together in series, and the optical multiplexor/demultiplexor takes out light from between the pre-stage optical amplifier section and the post-stage optical amplifier section of each of the respective optical amplifiers. By having such a construction, light signals of an appropriate power are input to the wavelength dispersion compensator. Therefore, the occurrence of nonlinear optical effects and degradation of the optical SN ratio is suppressed.
Moreover, as a specific construction for the respective optical amplifiers, a variable optical attenuator may be provided between the pre-stage optical amplifier section and the optical multiplexor/demultiplexor. Furthermore, with the variable optical attenuator, preferably the light attenuation amount is controlled so that the power of the light output from the post-stage optical amplifier section becomes a fixed level. Preferably, the gains of the pre-stage optical amplifier section and the post-stage optical amplifier section are controlled to be constant.
In addition, preferably, the wavelength dispersion compensator is a dispersion compensating fiber, and the optical multiplexor/demultiplexor has two optical multiplexing/demultiplexing devices respectively connected to both end portions of the dispersion compensating fiber. Lights of adjacent wavelength bands of the lights first taken out from the respective optical amplifiers are respectively input to the dispersion compensating fiber via the different optical multiplexing/demultiplexing devices. Moreover, the respective optical multiplexing/demultiplexing devices may be optical circulators.
With such a construction, the signal lights of adjacent wavelength bands are respectively input from the respective end portions of the dispersion compensating fiber via different optical multiplexing/demultiplexing devices, and propagated in mutually different directions inside the dispersion compensating fiber. As a result, the situation where signal light of large power is concentrated at a specific portion of the dispersion compensating fiber is avoided, and the propagation directions of the lights of adjacent wavelength bands are opposite. Therefore, the probability of the occurrence of nonlinear optical effects in the dispersion compensating fiber is further reduced.
Furthermore, objects of the present invention are achieved by providing an apparatus which demultiplexes light into a plurality of wavelength bands according to optical wavelength and respectively amplifies the demultiplexed lights of the respective wavelength bands with a plurality of optical amplifiers, and then multiplexes the amplified lights. The apparatus further comprises a first wavelength dispersion compensator for compensating for wavelength dispersion with a dispersion amount for a previously set wavelength as a reference, an optical multiplexor/demultiplexor which first takes out and multiplexes lights of respective wavelength bands sent to the respective optical amplifiers and then inputs the multiplexed light into the first wavelength dispersion compensator, and also demultiplexes the light output from the first wavelength dispersion compensator into the respective wavelength bands and returns the demultiplexed lights to the respective optical amplifiers. A second wavelength dispersion compensator separately compensates for the wavelength dispersion which is not completely compensated for by the first wavelength dispersion compensator, for each respective wavelength band.
With such a construction, in the case where wavelength dispersion for the respective wavelength bands cannot be compensated for by a single wavelength dispersion compensator, a first wavelength dispersion compensator for compensating for wavelength dispersion with a dispersion amount for a previously set wavelength as a reference is provided, and a second wavelength dispersion compensator for separately compensating for the wavelength dispersion which is not completely compensated for by the first wavelength dispersion compensator for each respective wavelength band is provided. In this way, an optical amplifier which can perform wavelength dispersion compensation with respect to the respective wavelength bands is realized with a comparatively simple construction.
Objects of the present invention are also achieved by providing an optical communication system for multiply repeating and transmitting wavelength division multiplexed signal light using a plurality of optical amplifier repeaters connected in series via an optical transmission path. The plurality of optical amplifier repeaters are optical amplifiers which divide the light into a plurality of wavelength bands according to optical wavelength, and respectively amplify the demultiplexed lights of the respective wavelength bands with a plurality of optical amplifiers and then multiplex the amplified lights. The plurality of optical amplifier repeaters have first and second constructions.
The optical amplifier repeater of the first construction includes a plurality of optical amplifiers for amplifying a plurality of wavelength bands, respectively. A wavelength dispersion compensator compensates for wavelength dispersion of the wavelength division multiplexed signal light with a dispersion amount for a previously set wavelength as a reference. An optical multiplexor/demultiplexor takes out and multiplexes lights of respective wavelength bands sent to the respective optical amplifiers and then inputs the multiplexed light into the wavelength dispersion compensator, and also demultiplexes the light output from the wavelength dispersion compensator into the respective wavelength bands and returns the demultiplexed lights to the respective optical amplifiers.
The optical amplifier repeater of the second construction includes a plurality of optical amplifiers for amplifying a plurality of wavelength bands, respectively. A first wavelength dispersion compensator compensates for wavelength dispersion of the wavelength division multiplexed signal light with a dispersion amount for a previously set wavelength as a reference. A second wavelength dispersion compensator separately compensates for wavelength dispersion which is not completely compensated for by the first wavelength dispersion compensator, for each respective wavelength band. Preferably, the first construction optical amplifier repeater and the second construction optical amplifier repeater are positioned alternately one after the other.
With such a construction, when the wavelength division multiplexed signal light, which is multiply repeated and transmitted by the plurality of optical amplifier repeaters, passes through the optical amplifier repeater of the first construction, the wavelength dispersion characteristics of the optical transmission path are compensated for by one wavelength dispersion compensator. With this wavelength dispersion compensation, the dispersion amount for the previously set wavelength is made a reference, and sufficient dispersion compensation is not performed for all of the respective wave bands. Therefore, when the wavelength division multiplexed signal light passes through the optical amplifier repeater of the second construction, the wavelength dispersion which has not been completely compensated for is separately compensated for each respective wavelength band. As a result, the wavelength dispersion characteristics of the optical transmission path can be compensated for all of the plurality of optical amplifier repeaters. Consequently, since the optical amplifier repeater of the first construction has a simple construction, it is easy to realize an optical communication system incorporating a wavelength dispersion function.
Furthermore, objects of the present invention are achieved by providing a wavelength dispersion compensation method in which wavelength division multiplexed signal light is demultiplexed into a plurality of wavelength bands according to wavelength. The lights of adjacent wavelength bands of the demultiplexed lights are respectively input from different end portions of the dispersion compensating fiber, and the lights of respective wavelength bands respectively output from the respective end portions of the dispersion compensating fiber are multiplexed.
With such a construction, the lights of adjacent wavelength bands are respectively input from the different end portions of the dispersion compensating fiber and propagated in mutually different directions inside the dispersion compensating fiber. As a result, the situation where signal light of large power is concentrated at a specific portion of the dispersion compensating fiber is avoided, and the propagation directions of the lights of adjacent wavelength bands are opposite. Therefore the probability of the occurrence of the nonlinear optical effect in the dispersion compensating fiber is further reduced.